Vasa Previa Depicted by Simple Clear Flow

INTRODUCTION

The recent incidence of vasa previa is 0.08%.¹ An accurate antenatal diagnosis of vasa previa is the most important to reduce perinatal fetal death.^2,3 However, accurate prenatal diagnosis of vasa previa using conventional color Doppler is still controversial.⁴

Simple Clear Flow is a novel flow imaging mode based on power Doppler ultrasound. It can optimize ultrasound scanning and image signal processing for small blood flow. Simple Clear Flow can be set at a higher line density than the power Doppler mode to visualize small blood flow. Simple Clear Flow achieves high resolution and sensitivity by adjusting some image signal processing such as spatial smoothing and line density to achieve appropriate signal-to-noise ratio even with a few repeats of transmission and reception. Small blood flow tends to be slow with weak signals. However, SCF can detect even small and slow blood flow more clearly by improving the resolution and sensitivity. It also maintains almost the same frame rate as the power Doppler mode. In this report, we present our experience of diagnosing vasa previa using SCF at 32 weeks and 1 day of gestation.

CASE DESCRIPTION

A 32-year-old pregnant Japanese woman, G (1), P (0), was reexamined using transvaginal SCF at 32 weeks and 1 day of gestation due to suspected vasa previa based on a previous routine obstetric examination at 30 weeks and 1 day of gestation using conventional color Doppler (Voluson P8, GE Healthcare Japan, Tokyo, Japan) (Fig. 1). In this case, placenta marginalis was suspected at 18 weeks and 3 days of gestation. Two-dimensional sonography (SONOVISTA GX30, KONICA MINOLTA, Tokyo, Japan) with a transvaginal probe (EC9-3, 3–9 MHz) clearly showed two small tubular structures at the periphery of the low-lying placenta near the internal cervical os (Fig. 2). Simple Clear Flow revealed that these structures were small velamentous blood vessels at the periphery of the posterior low-lying placenta near the internal cervical os (Fig. 3). Pulsed Doppler ultrasound revealed that the lower vessel was a vein and another vessel was an artery with arterial blood flow consistent with the fetal heart rate (Fig. 4).

At 37 weeks of gestation, emergency cesarean section was performed due to irritable uterine contractions, resulting in a viable, single female newborn weighing 2,752 g, with a height of 49 cm. The Apgar scores were 9 (1 minute) and 10 (5 minutes), and the umbilical artery blood pH was 7.3. Placental examination showed velamentous insertion of the umbilical cord with aberrant vessels (Fig. 5). The mother and neonate followed a favorable course after delivery.

DISCUSSION

There are two novel modalities: superb microvascular imaging and SlowflowHD, to detect slow blood flow in peripheral small vessels of the fetus and placenta.^5–9 Simple Clear Flow is another modality to depict small and slow blood flow. In this study, SCF clearly showed aberrant velamentous umbilical vessels near the cervical os compared with conventional color Doppler ultrasound. Especially, SCF could show small blood vessels without blooming. Moreover, SCF could be used with a transvaginal probe and a high-frequency transducer (3–9 MHz). To the best of our knowledge, this is the first report of diagnosing vasa previa using transvaginal SCF. Transvaginal SCF may be a useful diagnostic modality for the clear depiction of aberrant umbilical vessels near the cervical os in clinical practice.

In our clinic, two cervical length measurements were performed at 18–21 and 28–31 weeks of gestation. Consequently, placenta marginalis was suspected based on the first scan, and vasa previa was suggested based on the second scan. Vasa previa is usually defined as cord vessels present in the membranes covering the internal cervical os.¹⁰ In our case, however, aberrant cord vessels ran horizontally within 2 cm of the internal cervical os, not covering the os. In another definition, the distance between the internal cervical os and aberrant vessels should be less than 2 cm.¹¹ Therefore, cross-examination using transvaginal SCF around the internal cervical os may be mandatory for the accurate prenatal diagnosis of vasa previa to prevent perinatal fetal loss.

REFERENCES

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